[Radiance-general] Randance and different sky descriptions

[Zack Rogers]

Hey Rob,

So, as expected, Perez returns roughly the same illuminance fed to it.  I
used the clearest noon I could find around equinox, with a 9.76:1 (dir
nor/diff) ratio (3-19 at 12pm in the Boulder TMY2 file).  I get 8360fc
where the global illum is listed as 8259fc, which I'd say is equivalent.
 Looking at the percent difference between the resulting Perez sky and
IESNA recommendation is interesting.  The perez sky has the north horizon
roughly 1.2x brighter, the south horizon about .9X as bright, and the
circumsolar reqion about ~1.3x brighter.  The ratio image is here:
https://dl.dropbox.com/u/14937994/perezdivSPOT_fls.png  (this is the perez
sky divided by the IESNA sky for a sunny equinox at noon in Boulder, CO)

Cheers,
Zack

On Thu, Feb 28, 2013 at 3:13 PM, Guglielmetti, Robert <
Robert.Guglielmetti at nrel.gov> wrote:

> Wow, thanks for this investigation, Zack! Replies below.
>
> On 2/28/13 1:58 PM, "Zack Rogers" <zrogers at daylightinginnovations.com
> <mailto:zrogers at daylightinginnovations.com>> wrote:
>
> Here are the numbers I get for a Boulder, CO equinox clear and sunny sky
> at noon from TMY2 weather data, IES_gensky, and gensky
> Weather data = 7,893fc
> IES_gensky (5,000elev) = 9,416fc
> IES_gensky (0) = 8212fc
> gensky = 5823fc
>
> Wow, that's quite a spread. Might be interesting to throw in an "ideally
> clear" day using the Perez model and see where that ends up on this
> continuum -- and then go from there, because clearly this should be
> discussed further.
>
> You should be able to find an ideally clear direct normal/diffuse
> horizontal pair of values hovering around the Equinox for Boulder in the
> TMY/EPW set. When you see something like a 10:1 ratio between the direct
> normal and the diffuse horizontal irradiance, you can bet you're looking at
> a cloudless sky — certainly an unobscured sun, and vice-versa for a
> Portland/Seattle (er, overcast) sky.  See this link for example:
>
> https://www.dropbox.com/s/ri505ka29rr6knq/perez_skies.pdf
>
> This is for Summer Solstice, but you get the idea. Somewhere near the
> Equinox there should be a day where this 10:1 ratio is evident, and you
> could use that day and value pair with gensky, IES_gensky, and gendaylit
> and see what you get. Since gensky can also take these values as input (and
> presumably so can IES_gensky but I'm not certain), this might give us more
> of an "apples to apples" comparison. Also when using direct input of
> irradiance, you obviously don't want to use the cool elevation correction
> in IES_gensky, since the actual elevation of the instrumentation handles
> this for you. =)
>
> - Rob
>
>
>
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-- 
Zack Rogers, P.E., LEED AP BD+C
Daylighting Innovations, LLC
211 North Public Road, Suite 220
Lafayette, CO 80026
(303)946-2310
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